'88 Countdown: 3, 2, 1, Leap Second, 0

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As it happens, 1988 will arrive a little late this year - one second late, to be exact - and thereupon hangs a tale.

By international agreement, the world's timekeepers, in order to keep their official atomic clocks in step with the earth's irregular but gradually slowing rotation, have decreed that a ''Leap Second'' be inserted between 1987 and 1988.

So at precisely 11:59:60 P.M. on New Year's Eve, there will be a one-second void before the onset of 12:00:00 A.M. New Year's Day, and giddy revelers in New York City, across the nation and around the world will experience a strange moment between the years.

''It will be a moment that doesn't belong to anything - a glimpse of infinity - because it's outside regular time,'' said Tama Starr, whose company, the producer of the Times Square ball-lowering ceremony for 80 years, plans a spectacular one-second light show for the occasion.

The extra second, ordered by the world's nominal timekeeper, the Bureau International de l'Heure in Paris, will be marked officially at midnight Thursday in Greenwich, England, the home of what is popularly known as Greenwich Mean Time - Coordinated Universal Time to the cognoscenti - the standard for the planet. And around the world, to satisfy the requirements of navigators, communications organizations and scientific groups, 150 official atomic clocks will be adjusted at local times corresponding to midnight in Greenwich. The master clock at the United States Naval Observatory in Washington D.C. will be adjusted at 6:59:60 P.M., or 23:59:60 G.M.T. For Those in Reality

But unofficially - and for the sake of those who experience reality through television and through thronging, horn-tooting extravaganzas like the New Year's Eve celebration in Times Square - the moment will be marked at midnight local time in cities and towns across the nation.

For many years, the Bureau International de l'Heure in Paris has served as the clearinghouse and center for earth rotation studies and Leap-Second announcements. But starting in January, the United States Naval Observatory will join with the Paris-based organization in the new International Earth Rotation Service, which will announce all subsequent leap seconds.

The need for a Leap Second, like the need for a Leap Year Day every four years, arises because of differences between the time as recorded on our clocks and the time as recorded by the rotation of the earth and its revolution around the sun. Earth's Slowdown

Atomic clocks accurate to one-billionth of a second per day have been keeping the official time since the 1950's. With such precision, the clocks are always at variance with time as measured by the earth's movement. A Leap Year Day is needed because it takes the earth 365.25 days to orbit the sun and an extra day is accumulated every four years.

For various reasons - the sloshing molten core, the rolling of the oceans, the melting of polar ice and the effects of solar and lunar gravity - the planet rotates on its axis at irregular rates, and on average has been slowing down by about one-thousandth a second per day.

Thus, every few years a full extra second is accumulated on the clocks. So periodically, to get the clocks back in step with earthly rotation, a leap second has been inserted to remove the extra second from the clocks. Without much fanfare, it has happened 14 times since 1972, most recently in June 1985.

Not to be outdone by mere celestial events, the producers of this year's Times Square extravaganza, in which a lighted ball atop One Times Square is lowered down a flagpole to mark the new year, have promised a spectacular light show to coincide with the big non-moment. Countdown to '88

With an anticipated 300,000 people jamming the square and some 100 million watching on television around the nation, the ball - actually a 200-pound, 6-foot globe in the shape of a Big Apple illuminated with red lights - is to begin its one-minute descent at 11:59:00.

''The countdown will be 5, 4, 3, 2, 1, Leap Second, Zero,'' said Ms. Starr, executive vice president of Artkraft Strauss Sign Corporation, which has performed the ceremony since the celebration of the New Year in 1908. At the final moment, as Leap Second arrives, several things will happen simultaneously, according to Ms. Starr.

White strobe lights inside the ball will flash on, creating a ''sparkling, twinkling'' effect, two rows of multicolored lights on the parapet atop the 24-story tower will blaze down on the surrounding crowds, and a spinning ball fitted with 20 powerful spotlights will be turned on, sending beams flashing into the sky and onto the crowd and the facades of nearby buildings.

''It will be the shortest running hit in Broadway's history,'' Ms. Starr said yesterday, waxing eloquent over prodigious candlepower, the meaning of a mere second and other universal truths.

''One second may seem insignificant,'' she said, ''but its cumulative effect is staggering. With a single second added to the lives of all 4.8 billion people on earth, a total of 153 1/2 years will be created at Leap Second - two entire lifetimes worth.'' And that's not all. ''The extra second is a chance for all of us to reflect on the opportunities that we have,'' Ms. Starr said. ''By lighting up the sky over Times Square with a cascade of light, we hope to show what can be accomplished in a single second. We are sending a message to everyone: Make the most of each precious second of your life.''

Dr. Dennis McCarthy, an astronomer at the Naval Observatory and chief of the branch that measures the earth's rotation, expressed the meaning of a second in another and perhaps more graphic way.

''A second,'' he said, ''is a relatively long amount of time. If you're flying a plane by instruments and you're off by one second, you're going to miss the runway by nearly one-fifth of a mile.''

A version of this article appears in print on , Section B, Page 1 of the National edition with the headline: '88 Countdown: 3, 2, 1, Leap Second, 0. Order Reprints | Today’s Paper | Subscribe